The invention relates to vertebral implants, and more particularly, to implants made from allograft bone, or other suitable materials, with dimensions and geometries suitable for placement between adjacent vertebral bodies.
Disease, advancing age, and trauma can lead to changes in various bones, discs, joints, and ligaments of the body. Some changes and trauma often manifest themselves in the form of damage or degeneration to a spinal disc. This condition often results in chronic back pain, which can be anywhere from mild to severe. This pain can sometimes be eliminated by spinal fusion in which two adjacent vertebral bodies are joined together after removing the intervening intervertebral disc. A prosthetic device is usually placed between the two adjacent vertebral bodies, in place of the removed disc, to fill the space left by the removed disc and to allow bone to grow between the two vertebral bodies.
The success or failure of the fusion can often depend upon the type and properties of the prosthesis that is placed between the adjacent vertebral bodies. Obviously, the prosthesis must be sufficiently strong to withstand the loads encountered in the spine, it must be biocompatible, and it should permit the ingrowth of bone to complete the fusion. Perhaps more importantly, the prosthesis must have properties and geometries that permit the prosthesis to remain fixed in the desired position, without movement due to shifting, rotation, or slippage of the prosthesis.
A variety of prosthetic implants are known to exist. For example, a variety of artificial intervertebral discs are disclosed in U.S. Pat. Nos. 5,893,889; 5,683,465; 5,674,294; 5,458,643; 5,306,309; 5,246,458; and 4,759,769. Other known implantable prostheses for such applications include fusion cages made from metals and/or synthetic materials.
Many prostheses can also be fashioned from allograft bone that is harvested from portions of long bone including the femur, humerus, tibia, fibula, ulna and radius. An example of an allogenic intervertebral implant is disclosed in U.S. Pat. No. 6,143,033.
Despite known prostheses for the fusion of adjacent vertebral bodies, there remains a need for additional prostheses that have properties and geometries suitable for secure and effective implantation within a patient""s spine.
An implantable bone graft is in the form of a solid annular body having opposed superior and inferior surfaces, and opposite anterior and posterior ends. The substantially central opening defined by the annular body extends between the superior and inferior surfaces and, during surgical implantation, the opening can be filled with a bone growth-promoting material. At least a portion of both the superior and inferior surfaces include a plurality of discrete protrusions or teeth. Each protrusion preferably has a base that is integral with one of the superior and inferior surfaces. Both the superior and inferior surfaces further include at least one region or band that is free of any protrusions. The protrusion-free band is preferably in a predetermined shape, such as rectangular, and is oriented in a desired direction. These bands preferably are centrally disposed on the superior and inferior surfaces and can be aligned so as to be parallel to an anterior-posterior axis of the body or angled with respect to the anterior-posterior axis of the body.
The body also includes a tip that is formed at the posterior end thereof. The tip is preferably formed by converging portions of superior and inferior surfaces. The tip may extend over a distance that represents ten to fifty percent of the anteriorposterior depth of the body.
In one embodiment, both the superior and inferior surfaces are canted such that the thickness dimension of the graft is greater at the anterior end than at the posterior end. Alternatively, both the superior and inferior surfaces can be oriented parallel to the anterior-posterior axis of the body. In yet another embodiment, the superior surface is oriented parallel to the anterior-posterior axis, but the inferior surface is angled such that the thickness of the graft is greater at the anterior end than at the posterior end. Both the superior and inferior surfaces can be substantially linear or, alternatively, they can have a dome-like profile, while still maintaining any inclination of the surface from the anterior to the posterior end.